In an air flow rate measuring device disclosed in JP-A-11-23336, a hole is formed between an inlet and an outlet of a bypass passage provided with a heater element. The hole communicates the bypass passage with a main air passage. Besides, pressure difference causes through the hole between the side of the bypass passage thereof and the side of the main air passage thereof, so that air flows from the bypass passage to the main air passage through the hole. Thereby, flow velocity increases at the heater element in the bypass passage, so that measuring accuracy is enhanced even when a flow rate is low.
An intake air quantity is measured in an internal combustion engine in a conventional structure, in which a bent portion is formed midway a bypass passage to change air in flow direction, so that a length of the bypass passage is adjusted. Thereby, measurement error, which is caused by influences of intake pulsation, is decreased. Here, as disclosed in JP-A-11-23336, a hole is provided midway a bypass passage, so that air, which flows through the bypass passage, partially flows into a main air passage through the hole, so that the same effect as when the bypass passage is modified in length may be produced.
In a bent portion provided in a bypass passage, a hole may be arranged in the same position relative to both a path of air flowing on the inside of the bent portion and a path of air flowing on the outside of the bent portion. However, in this structure, effect of compensation of intake pulsation becomes different between the respective paths from each other, and accuracy of compensation may become insufficient.